How do I scan riddler question marks?

I've played through all of Arkham Asylum and I still don't remember how to do this - I see question marks all over the place in detective mode, but holding down "L2" doesn't seem to scan them no matter how I try to align it. (I don't even get a "misaligned" message.) How do you scan the question marks?

Solution 1:

You need to be able to interrogate Riddler informants. The scan feature unlocks after you question the first one, I think.

http://community.batmanarkhamcity.com/forums/showthread.php/6563-Can-t-Make-the-Riddler-s-Riddles-Work...

Solution 2:

I had the same problem when i started the game! Once you enter one of the first areas in the story mode, the "scan" option opens up to you. You're prompted to scan two trophies locked in an elevator shaft so you can come back to them. Once you do that, you'll be able to scan whenever you'd like. Confusing, but that's how it goes.